EfiO2Meter Theory of Operation
Bosch LSU 4.x O2 Sensor Theory of Operation
In very simple terms ... the Wide Band LSU 4.x sensors are not simple at all. They consist of 3 basic items.
1) Nernst Cell : or Narrow Band O2 sensor which, once at operating temperature, produces the usual voltage of 0 mV for lean to 900 mV for a rich air fuel mixture exhaust gas.
If the AF mixture is at the ratio where all O2 is consumed by the fuel in accordance of the correct chemical equations, the voltage put out is right at the middle at 450 mV.
The inner resistance Ri of the sensing element at cold or ambient temperature is in the Mega Ohm and no voltage can be produced. The hotter (800 degC) the sensor gets, the lower the inner resistance and the sensor produces the above described voltages.
2) Ion Pump Cell : The ion pump cell is simply speaking internally connected to the Nernst cell and the direction as well as the amount of current going into or is being drawn out of this ion pump current affects the switch-point of the Nernst cell.
Another way of describing would be that, a lean or rich mixture forces the Nernst cell to produce a voltage deviation off the 450 mV.
This can be counteracted by supplying a positive or negative ion pump current.
Once this pump current is adjusted (closed loop controlled) so that the Nernst cell reached again it's equilibrium of 450 mV, the pump current can now be used to plot it at a defined conversion of Ion Pump Current to Lambda Value curve.
3) Sensor Heater : Since this Ion Pump Current to Nernst conversion ratio is sensor temperature dependent it is necessary to provide a constant temperature control for the sensor.
This carries another possible problem. The sensor operating specs are at 800 degC. If the sensor location is now to close to the engine exhaust port is is very likely that this 800 degC will be exceeded, even if the PID heater control look turns off all heating.
Once we have a chance to do a study on stock O2 sensor locations we will post the findings.
We will not elaborate longer about the sensor's theory of operation, cause the web will for sure have after a bit browsing plenty available to read up on.
Bosch CJ125 sensor Interface IC Theory of Operation
Well, here the story goes ...
The schematic below is a very simplified schematic explaining part of the CJ125's operation.
U2 provides a steady 2.5 V virtual ground for the sensor. This allows an easy positive and negative Ion Pump Current generation.
The 450 mV VM based voltage source carries the 450 mV reference voltage the Nernst output voltage will be compared against by the differential input Operational Trans Conductance amplifier U1 (integrator).
The output of this Nernst Cell voltage controlled OTA output is being fed into pin IA of the sensor.
The voltage flows from sensor pin IA to pin IP over a 61.9 Ohm current measurement resistor. Sensor pin IP finally feeds this current into the Ion Pump Current.
The voltage drop across this 61.9 Ohm resistor is the input into the Ion Pump Current amplifier U3 with a displayed gain of 8 (Lambda Range 0.650 on up) . This gain is also switchable to 17 if the Lambda range below 0.750 is not of interest.
UA is the CJ125 output voltage proportional to the Ion Pump Current.
The sensor has a calibration resistor located in the sensor connector which is connected in parallel to the Ion Pump Current measurement resistor.
This parallel calibration resistor makes sure that the voltage drop across the current measurement resistors (controller and sensor based) on all sensors is in accordance to the LSU 4.x provided sensor current to Lambda conversion table.
There are internal calibration circuits (not shown) in the CJ125 facilitating shorting the inputs of U3 to determine the zero input UA reference voltage.
The heater control based upon Ri measurements of the Nernst Cell is a separate complex issue which is of importance, but secondary with respect to, that the instrument measures the Ion Pump Current UA to determine the air fuel ratio.
That's about it unless we come up with more.
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